1. Field of the Invention
The present invention relates to index table assemblies.
2. Description of the Related Art
Index table assemblies including a rotary table, a frame separated from the rotary table in the direction of a rotational axis of the rotary table, and two bearings disposed between the rotary table and the frame, the bearings receiving forces in opposite directions along the rotational axis, are known in the art (refer to, for example, Japanese Patent No. 2930889, FIG. 1 on page 4). In such an index table assembly, the frame supports the rotary table against an external force applied to a workpiece-receiving surface of the rotary table and the weight of the rotary table so as to maintain the workpiece-processing accuracy.
In an index table assembly disclosed in the above-mentioned patent, a frame has a through hole in which a cylindrical portion of the main body of a rotary table and a cylindrical portion of a bearing support extend from opposite directions. The cylindrical portions are fixed to each other with bolts to form a core of the rotary table. A first thrust bearing is positioned between a surface of the frame on the same side as a workpiece-receiving surface of the rotary table and the main body of the rotary table, and a second thrust bearing is positioned between a surface of the frame on the side opposite to the workpiece-receiving surface of the rotary table and a flange of the bearing support. The first thrust bearing receives a force applied in a direction from the workpiece-receiving surface of the rotary table toward the frame along the rotational axis, and the second thrust bearing receives a force applied in a direction opposite to the direction from the workpiece-receiving surface of the rotary table toward the frame along the rotational axis.
An assembly process in which the rotary table is attached to the frame when the index table assembly is manufactured or overhauled will be described below. Before assembly, the bearing support is detached from the rotary table. First, the first thrust bearing is installed onto the frame from the side of the workpiece-receiving surface. Then, the main body of the rotary table is installed onto the first thrust bearing from the same side such that the cylindrical portion of the main body of the rotary table is inserted into the through hole. Then, the second thrust bearing is installed onto the frame from the opposite side, that is, from the side opposite to the workpiece-receiving surface. Then, the bearing support is installed onto the second thrust bearing from the same side as the second thrust bearing such that the cylindrical portion of the bearing support is inserted into the through hole. Then, the cylindrical portion of the bearing support and the cylindrical portion of the main body of the rotary table are fixed to each other with the bolts, and the process of attaching the rotary table to the frame is completed.
In this index table assembly, the main body of the rotary table and the first thrust bearing are installed from the side of the workpiece-receiving surface and the second thrust bearing and the bearing support are installed from the side opposite to the workpiece-receiving surface. Therefore, the frame must be turned over at some point, and this must be performed carefully since there is a risk that the first thrust bearing and the main body of the rotary table, which are already installed on the frame, will be displaced due to their own weights.
In addition, in the above-described index table assembly, the cylindrical portion of the main body of the rotary table and the cylindrical portion of the bearing support are fixed to each other with a shim interposed therebetween, and a suitable preload is applied to the first and second thrust bearings by adjusting the thickness of the shim. When a suitable preload is applied to the thrust bearings, no play is provided along the rotational axis and the rotary table is prevented from being displaced along the rotational axis due to the external force, so that the workpiece-processing accuracy is maintained. In addition, the thrust bearings are prevented from being worn in a short time due to excessive preload and the rotational resistance is set low.
When the bearing support is attached to the main body of the rotary table, the preload applied to the thrust bearings is adjusted by changing the thickness of the shim while rotating the frame, which is placed on top of the rotary table at this time, and monitoring the rotational resistance of the frame. However, a load applied on the thrust bearings in the normal state in which the rotary table is on top of the frame is different from that in the state in which the heavy frame is on top of the rotary table, and therefore the rotational resistance of the thrust bearings is also different between the two states. Accordingly, the rotational resistance of the rotary table generated when the rotary table is rotated in the normal state must be calculated while rotating the frame, and it is difficult to apply a suitable preload to the thrust bearings.
In order to eliminate the step of turning over the frame, the rotary table may be attached while the frame is placed such that the rotational axis of the rotary table extends horizontally. However, in this case, the main body of the rotary table and the thrust bearings cannot be installed onto the frame using their own weights, and therefore the task of installing them is difficult. In addition, when the preload is adjusted while rotating the rotary table and monitoring the rotational resistance of the rotary table, the load of the rotary table is not applied to the thrust bearings. Therefore, similar to the above-described case, the rotational resistance of the rotary table in the normal state must be calculated, and it is difficult to apply a suitable preload to the thrust bearings.